1. Field of the Invention
The present invention disclosed in this specification relates to a doping method using an ion doping apparatus which does not require mass separation of generated ions and a method of manufacturing a field effect transistor using the doping method.
2. Description of the Related Art
In a manufacturing process of a semiconductor element such as a field effect transistor, when a donor impurity or an acceptor impurity is added into a processing object such as a semiconductor film formed over a substrate having an insulating surface or a semiconductor substrate, an ion implantation apparatus or an ion doping apparatus is used. An ion implantation apparatus is a mass-separation type apparatus in which an unnecessary ion species can be separated by using a mass separator and in which a processing object placed in a treatment chamber can be subjected to only a desired ion species. Therefore, the dose amount of a desired ion species can be precisely controlled.
On the other hand, since a mass separator is not included in an ion doping apparatus, the ion doping apparatus is a non-mass-separation type apparatus in which a processing object placed in a treatment chamber is irradiated with all ions included in an ion beam (hereinafter- referred to as total ions in this specification) which is extracted from plasma generated in an ion source. Accordingly, the doze amount is counted by not only a desired ion species but also total ions including an unnecessary ion species, which makes it difficult to precisely control the doze amount of a desired ion species.
Hereinafter, an ion implantation apparatus refers to an apparatus with a mass separator, and an ion doping apparatus refers to an apparatus without a mass separator in this specification.
As a source gas, for example, PH3 (phosphine) diluted with hydrogen is used in a case of using phosphorus as a donor, and B2H6 (diborane) diluted with hydrogen is used in a case of using boron as an acceptor. In an ion source, the source gas is separated into positive ions and electrons; in other words, the source gas is ionized to generate plasma. Then, an ion beam is extracted from the plasma. Since the source gas includes hydrogen as described above, a large amount of hydrogen ions is included in the generated plasma. This hydrogen ion is an unnecessary ion species.
Since the dose amount is counted by total ions including the hydrogen ions in the ion doping apparatus, a proportion of a desired ion species in total ions is varied depending on a condition of plasma even if the dose amount of total ions is not changed. In this case, the dose amount of only a desired ion species is forced to change.
In addition, the precise control of a concentration of boron in a semiconductor substrate or a semiconductor film is required in doping a portion where a channel region is formed with boron as an impurity at a low concentration, that is to say, in channel doping, in order to control a threshold voltage Vth of a field effect transistor. However, the ion implantation apparatus is sometimes used only in a step of channel doping since the precise control is difficult to be performed with the ion doping apparatus.
Among the ion doping apparatuses, there is an ion doping apparatus including a mass spectrometer. By using the mass spectrometer, a proportion of a desired ion species can be monitored. However, when doping of boron at a low concentration is performed as in the case of channel doping, there is a problem in which ions of a compound including boron, in other words, a desired ion species is not detected by the above mass spectrometer.
The invention described in Reference 1 focuses on that a peak with high intensity due to H3+ ions is observed by using a mass spectrometer (referred to as E×B) equipped in an ion doping apparatus, even in such a condition in which doping is performed with an impurity at a low concentration (Reference 1: Japanese Patent Laid-Open No. 2004-39936). In other words, the invention attempts to control the dose amount of boron by finding a correlation between the peak intensity due to H3+ ions and a concentration of boron in a processing object, which has been measured by SIMS (secondary ion mass spectrum) analysis.
However, it is found that even when the invention described in Reference 1 is used, a concentration of boron in the processing object is not stable and the variation is not small in the condition of doping with an impurity at a low concentration. Since the dose amount of boron cannot be controlled precisely, the improvement of the above invention is required.